Sulfur Plasma Lamp

ABSTRACT

A sulfur plasma lamp has a lamp envelope of transparent or translucent glass or ceramic material. At least two silicon carbide electrodes are hermetically sealed with the lamp envelope and in contact with an interior of the lamp envelope. A quantity of sulfur within the interior of the lamp envelope is sufficient to create a sulfur plasma upon excitation. A buffer gas within the interior of the lamp envelope enables initial discharge and heating of the interior of the lamp envelope to excite the sulfur into a plasma state. More than two electrodes may be provided, and an electrical potential is created between different pairs of the electrodes at different times, thereby inducing stirring of the plasma upon excitation of the material into a plasma state.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application claims the benefit of U.S. ProvisionalApplication 62/463,702, filed Feb. 26, 2017, the entire disclosure ofwhich is hereby incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to plasma lamps and their manufacture anduse, and more specifically to sulfur plasma lamps with non-oxide ceramicelectrodes such as silicon carbide electrodes.

BACKGROUND

High-efficiency light sources that offer a balanced and completeemission spectrum (white light) are used in the consumer, agricultural,and industrial market place. There are certain options available tothese markets that meet the criteria of the light source being veryefficient and offering a full spectrum light. Typical white lightsources include high-pressure mercury lamps; metal halide lamps, highpressure xenon plasma lamps, or less frequently, sulfur plasma lamps.Xenon lamps are non-toxic but may emit ultraviolet light as well asvisible light, and include xenon incandescent lamps used for automobileheadlights, which do not emit ultraviolet light because the glass isengineered to filter out the ultraviolet, as well as xenon dischargelamps that can be expensive and that operate at very high pressures andcan be dangerous if breached. In halide-containing lamps, the tungstenelectrode material may tend to vaporize during operation of the lamp,but a recycling process known as the halogen cycle occurs in which thesmall amount of vaporized tungsten re condenses on the filament,prolonging life of the lamp.

Sulfur plasma lamps emit almost no ultraviolet light, which mightotherwise waste energy, age plastic, or damage biological tissue, as canhappen with mercury, tungsten filament lamps, or xenon lamps. Sulfurplasma lamps systems tend to be more complex and bulkier than, forexample, metal halide lamps having just a small envelope withelectrodes. Unlike metal halide lamps, in which bromides, chlorides, andmercury in the lamp emit a combination of atomic photoemission, sulfurplasma lamps produce molecular photoemission due to the presence ofdimers and/or more complex sulfur molecules (S3, S8, etc.). Becauseplasma is very high energy the molecular state is dynamic andcontinuously changing and the molecular emission is over a broadspectrum, in contrast to the distinct spectral lines of atomic emission,thereby providing greater spectral uniformity over the 400-700 nanometerrange of white light. The continuous sulfur plasma emission spectrumoutput nearly matches sunlight perfectly. Sulfur is benign andinexpensive with little environmental impact, and sulfur plasma lampsneed not incorporate hazardous mercury as is used in other technologies.Sulfur plasma lamps are especially useful for providing light to plants.Plants, such as bananas, grown in indoor greenhouses with sulfur plasmalamps can yield more vegetation and fruit than plants grown with othertraditional kinds of white light sources. Sulfur plasma lamps can beoperated at high power levels and thereby illuminate very large spaces.

Sulfur plasma lamps currently available typically have no electrodes andare shielded and mounted in a microwave or EMF resonance cavity. Onereliable method that has been found is to excite a sulfur plasma byusing a microwave waveguide and resonance cavity system. A mesh is usedin conjunction with the resonance cavity, similar to a microwave oven.The resonance cavity must be carefully tuned, and a magnetron isnecessary for this method, but this method can provide a powerful lampwith a long lifetime, although there is a minimum power that isdependent on the type of magnetron that is available. Alternatively,microwave sources that are solid-state chips may be used instead ofmagnetrons in connection with sulfur or mercury plasma lamps. Anothermethod uses a radio-frequency coil in near proximity to or around theperimeter of a hermetically sealed envelope containing sulfur and abuffer gas, as is done in connection with the Icetron induction lightsystem. The use of a radio-frequency coil in near proximity to thesulfur induces excitation of the sulfur by way of electromagneticinduction. This technique involve complex technology that can be costly,and efficiency is limited due to the limits of coupling efficiencybetween the induction source and the contents of the lamp. Sulfur plasmalamps can be driven with electrodes if the lamp is spun at high speed tocreate centrifugal force that keeps sulfur away from the electrodes toprevent chemical reaction between the electrodes and sulfur. An attempthas been made to use metal electrodes coated with metal oxides fordriving sulfur plasma lamps, with the intention that the metal oxidecoating will prevent chemical reaction between the metal and the sulfur,but smooth and consistent operation of the lamp and the overall lifetimeof the lamp is dependent on the oxide layer not being breached due tocyclical heating, aging, pinholing by passage of electricity in a smallconcentrated location, cracking due to thermal expansion mismatchbetween the oxide layer and metal, etc., and reliable and consistentdischarge depends on the degree of predictability of electric currentpassing through the oxide layer.

SUMMARY

The invention provides a sulfur plasma lamp in which the plasma isdriven directly with electrodes as is accomplished with standard lightbulbs such as metal halide and mercury lamps. According to theinvention, it has been discovered that if silicon carbide is used as theelectrode material, the sulfur plasma can be driven reliably by theelectrodes, especially if the silicon carbide electrodes are doped toincrease electrical conductivity, and yet there will be no substantialchemical reactivity between the silicon carbide and the sulfur, which isotherwise very chemically reactive, especially in the plasma state.Doping of the silicon carbide can make it conduct electricity just aswell as metal. The silicon carbide exhibits long lifetimes and goodperformance and the ability to directly excite the sulfur to a plasmastate. Because the sulfur plasma lamp is driven by electrodes, it ispossible to operate the lamp over a substantial range of intensities.Sulfur plasma lamps according to the invention involve simple technologythat can be scaled small, with ease of manufacture, and the lamps can beused in a wide range of applications due to the wide range of powerintensities possible due to the nature of the technology.

According to one aspect of the invention, a sulfur plasma lamp includesa lamp envelope of transparent or translucent glass or ceramic material,and at least two non-oxide ceramic electrodes hermetically sealed withthe lamp envelope and in contact with an interior of the lamp envelope.A quantity of sulfur within the interior of the lamp envelope issufficient to create a sulfur plasma upon excitation. A buffer gaswithin the interior of the lamp envelope enables initial discharge andheating of the interior of the lamp envelope to excite the sulfur into aplasma state.

In certain embodiments, silicon carbide electrodes are used, which aren-doped. The buffer gas may include at least one of carbon dioxide,oxygen, nitrogen, nitrous oxide, argon, krypton, xenon, or, neon. Thelamp envelope extends outwardly from the interior of the lamp envelopeat least over a portion of the silicon carbide electrodes. The siliconcarbide electrodes may be hermetically sealed with the lamp envelope bya direct sealing of the lamp envelope with the silicon carbideelectrodes. Alternatively, the silicon carbide electrodes arehermetically sealed with the lamp envelope by a vacuum-tight epoxy,solder glass, or metallic solder at distances spaced from the interiorof the lamp envelope sufficient to avoid damage to the vacuum-tightepoxy, solder glass, or metallic solder from heat from the interior ofthe lamp envelope. Alternatively, electrical lead feedthroughs incontact with and extending outwardly from the silicon carbide electrodesare hermetically sealed with the lamp envelope. A grading glass orintermediate glass may be positioned between the silicon carbideelectrodes and the lamp envelope, the grading glass or intermediateglass providing a seal between the silicon carbide electrodes and thelamp envelope. The silicon carbide electrodes may have an undercut thatengages with material of the lamp envelope to create hermetic integrityfrom the silicon carbide electrode and the material of the lamp envelopepressing against each other upon heating. The silicon carbide electrodesmay be angled toward each other at substantially less than 180 degrees.There may be more than two silicon carbide electrodes.

According to another aspect of the invention, a method of manufacturinga sulfur plasma lamp as described above is provided.

According to another aspect of the invention, a plasma lamp includes alamp envelope of transparent or translucent glass or ceramic material,and at least three electrodes hermetically sealed with the lamp envelopeand in contact with an interior of the lamp envelope. A quantity ofmaterial within the interior of the lamp envelope is sufficient tocreate a plasma upon excitation. A buffer gas within the interior of thelamp envelope enables initial discharge and heating of the interior ofthe lamp envelope to excite the material into a plasma state.

The details of various embodiments of the invention are set forth in theaccompanying drawings and the description below. Numerous other featuresand advantages of the invention will be apparent from the description,the drawings, and the claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a sulfur plasma lamp withsilicon carbide electrodes, together with an inset showing detail of theinterface between an electrode and the lamp body.

FIG. 2 is a schematic representation of a sulfur plasma lamp in which avacuum-tight epoxy, solder glass, or metallic solder reinforces the sealbetween the silicon carbide electrode and the lamp body.

FIG. 3 is a schematic representation of a sulfur plasma lamp in whichelectrical feedthroughs are bonded to a hermetic molybdenum-to-glassseal.

FIG. 4 is a schematic representation of another embodiment of a sulfurplasma lamp in which electrical feedthroughs are bonded to a hermeticmolybdenum-to-glass seal.

FIG. 5 is a schematic representation of a sulfur plasma lamp in which agrading glass or intermediate glass achieves a hermetic seal between theelectrodes and the lamp body.

FIG. 6 is a schematic representation of a sulfur plasma lamp in whichthe electrodes have an inverted geometry, or undercut, formed in thebody of each electrode.

FIG. 7 is a schematic representation of a sulfur plasma lamp in whichthe electrodes have an inward-projecting feature formed in the body ofeach electrode.

FIG. 8 is a schematic representation of a sulfur plasma lamp in whichthe electrodes approach each other at an angle substantially less than180 degrees.

FIG. 9 is a schematic representation of a sulfur plasma lamp havingthree electrodes.

Identical parts are indicated by the same reference numerals. Indiceswith letters indicate different versions of the same element.

DETAILED DESCRIPTION

With reference to FIG. 1, the structure of a sulfur plasma lamp inaccordance with the invention includes a transparent or translucentenvelope 5 made of glass or ceramic, which has a plurality of electrodes6 hermetically sealed through the wall of envelope 5 from the outsideentering into the free volume within envelope 5. Envelope 5 may bespherical, cylindrical, planar, or any geometry in which there is freevolume within. Though not shown in FIG. 1, envelope 5 typically extendsoutwardly from the interior of the envelope at least over a portion ofelectrodes 6, as is illustrated in FIGS. 2-7, although the length ofsuch extension of envelope 5 may be only a millimeter or two. Theoptimal distance between electrodes 6 can be a function of voltageapplied to the sulfur plasma lamp, and a function of pressure, since aspressure goes up the voltage requirement may increase, for example.Electrodes 6 may be oriented directly opposed from one another ornearing each other at an angle substantially less than 180 degrees, asis shown in FIG. 8, which may be beneficial to project light as radiantflux onto an area of interest, with most of the radiant flux tending tocome out at the bottom of FIG. 8. Certain luminaires, which house thelamp, might work more efficiently if the electrodes approach each otherat less than 180 degrees. It is also possible to have three or moreelectrodes, as is shown in FIG. 9, in order to have a successiveexcitation discharge, which promotes rotating (stirring) of the sulfurplasma. If more than two electrodes 6 are used, a power supply can fireacross different pairs of electrodes at different times, therebyinducing stirring, which can be beneficial for homogeneity of theplasma, and beneficial during a cooling period so that averagetemperature is reduced to increase lifetime of the lamp. Switching canoccur at a rate of at least a couple of dozen hertz, up to the kilohertzor megahertz range.

The envelope material is constructed of any suitable glass or ceramic,and may be amorphous silica, aluminosilicate glass such as the type usedin automotive halogen lamps, borosilicate glass such as that used in labware, or a translucent or transparent aluminum oxide or zirconium oxide.Glass offers the convenience of being easy to form, easy to process, lowcost, and ease of automation of the processes of manipulating the glassto form lamps. Ceramics offer enhanced levels of chemical durability,and permeability to atmospheric gasses so that hydrogen and other gassesdo not tend to diffuse into the lamp.

The free volume of envelope 5 may vary from several cubic millimeters toseveral hundred cubic centimeters, and envelope 5 may be ellipsoidal,spherical, or cylindrical, or may have other geometries. The amount offree volume is limited by how much light is to be produced, becausethere is a relationship between volume and power of the lamp. Becausethe sulfur plasma lamp might operate under very high pressure, a morespherical geometry may he desirable because it promotes uniformdistribution of mechanical loading and endures high pressures. To obtaina good emission spectrum, it is desirable to operate at high pressures,as much as 10 and even 20 atmospheres, although pressures as low as ½atmosphere could also be used. Due to the relationship between pressureand dielectric breakdown, greater energy may be required at highpressure.

Within envelope 5 is a small amount of sulfur 3, which is solid at roomtemperature, prior to lamp ignition. The amount of sulfur containedwithin lamp envelope 5 may range from fractions of a milligram toseveral grams depending on the spectral emission output of the lampdesired and its operating temperature and lamp envelope volume. Thereshould be enough sulfur in the lamp that upon heating the desired vapordensity is achieved. The fill material within envelope 5 is not limitedto just sulfur 3, but may include additional materials such as seleniumor bromides, iodides, chlorides or various salts to further modify orshift the emission spectrum or promote a halogen-cycle-like effect thatcauses vaporized electrode material to be recycled back onto theelectrode. Sulfur has a somewhat green hue, and so it may be helpful toadd calcium bromide for additional red, or selenium, although seleniumcan be harmful. Also, bromides, chlorides, or any other metal orsemi-metal can introduce an element of toxicity. When sulfur plasmalamps are used primarily for plants, the presence of the green light isnot a substantial concern, but if the lamps are used for residential orautomotive purposes, the additional components may be desirable. Sulfurplasma provides good performance in that it yields a full spectrum, andin that the electrical characteristics of the lamp, such as voltage andcurrent, remain stable over a long period of time whereas other lampsmay tend to get dimmer as their resistance increases. The emissionspectrum of the lamp may be tuned by the quantity of sulfur per unitlamp volume, so that the lamp may be operated at a desired vaporconcentration, with operating temperature being a factor in the vaporconcentration. Denser plasma tends to absorb its own discharge and soaffects the spectrum of light that is emitted by the lamp.

The lamp also contains a buffer gas 4 for assisting with the initialdischarge of the lamp and initial warming period. Buffer gas 4 is usedto start the plasma discharge and then transfer energy to the sulfur,resulting in the sulfur turning into a gas and joining the plasma state.Thus, the warming period is necessary to evaporate cold condensed sulfur3 into a gaseous state, at which point it is excited by the electricaldischarge and enters the plasma state. In other words, it is necessaryto use a buffer that is a gas at room temperature so that discharge canstart and very quickly heat up and turn the sulfur to liquid and then togas. The voltage required to initiate discharge can be high. Typically,argon is used at a room-temperature pressure of a few torr to a fewhundred torr. A Penning mixture of gas can assist in achieving dischargemore easily due to the inclusion of a gas having a relatively lowionization potential, similar to the addition of a small amount ofkrypton gas to a neon gas lamp. Other gasses may be used, such as anyother inert or noble gas, especially krypton, xenon, or neon. Carbondioxide, oxygen, nitrogen, or nitrous oxide may serve as alternativegasses that help produce a spectrum that is altered in a beneficialmanner to be closer to white light due to addition of red light, mayassist in the discharge process by making the startup process easier,and may result in increased lamp lifetime. In particular, thesealternative gasses may be of assistance to lamp lifetime in that whensmall amounts of electrode material are being degraded, or broken down,these gases may promote re-condensation of the original electrodematerial back onto the electrodes rather than on the interior of thelamp envelope, similar to the well-known halogen cycle in tungstenfilament halogen lamps. Any combination of all the above-mentionedgasses will also work in a satisfactory manner with pressures asdescribed above. Chemical reactions will occur with the sulfurmolecules, but the environment is highly dynamic due to the energy ofthe system being so high, and so the molecules do not exist for a longtime. With cooling, there might be sulfur oxides, hydrogen sulfides,etc.

The methods of sealing n-doped silicon carbide electrodes 6 to the lampbody depend on the lamp envelope material selected for the application,the lamp size, and the operating conditions of the lamp. Hermeticsealing is needed to retain the light-emitting sulfur and buffer gasconstituents within lamp envelope 5. The method of sealing providing thebest simplicity of pressing of the silicon carbide electrodes to thelamp body is by a simple, direct glass-to-ceramic seal in which a glasswith a matched thermal expansion curve to the silicon carbide is used asthe material of lamp envelope 5. If an attempt is made to seal materialstogether having thermal expansion properties that are too far apart,cracking can occur. The silicon carbide is the ceramic of theglass-to-ceramic seal and the lamp envelope is the glass of theglass-to-ceramic seal. The most readily available and easiest source ofsuch glasses are the glasses that are designed to seal to molybdenum,which has a thermal expansion curve similar to silicon carbide, such as,for example, but not limited to, the aluminosilicate glasses used forproduction of automotive halogen headlamp bulbs. Examples of these glasstypes include GE180 glass, SCHOTT 8252 and SCHOTT 8253, Corning 1720.Other glass types are adequate for accomplishing this seal such asSCHOTT 8243 and SCHOTT 8250. The matching of glass to the siliconcarbide is similar to matching techniques used in connection with othertypes of electrode materials. Although all of these glasses matchsilicon carbide in expansion properties, silicon carbide is very inert,and so to get a hermetic seal, the glass has to wet the silicon carbide.The seals can be made by a traditional glassblowing technique. Accordingto a traditional heating and cooling process, the electrode may be inone chuck of a lathe and the glass tube in the other chuck, and then thetwo chucks are moved toward each other so that sleeving occurs, and thenthe sleeved connection may be heated with a glass-blowing torch, and agraphite paddle, similar to a spatula, can be used to press the glassdown onto the silicon carbide. Alternatively, a small piece of glassaround the silicon carbide may be melted to form a bead, and thentraditional glass-blowing processes may be used, either executed byhumans or by machine. Alternatively, automated pinch seals may be used,in which torches heat a glass tube around a silicon carbide rod, and atool is used to pinch the tube physically around the rod. The successrate of making the seals is not 100%, however, because there will alwaysbe some seals that fail to achieve a hermetic bond/seal. Accordingly, asis shown in FIG. 2, a vacuum-tight epoxy may be used as sealing material10 applied on the exterior of the ceramic-glass interface to complete infull or to enhance the reliability of the seal.

Alternatively, the materials of envelope 5 may also be fused silica, inwhich case a satisfactory seal is accomplished by use of a solder glass,grading glass, or metallic solder as sealing material 10 shown in FIG.2. In the instance where lamp envelope 5 is made of a ceramic, a solderglass or metallic solder is used as sealing material 10 to seal thesilicon carbide electrodes to aluminum oxide or zirconium oxide lampenvelope 5. Sealing material 10 reinforces the hermetic seal betweensilicon carbide electrodes 6 and lamp envelope 5 that occurs due toconstriction of the material of lamp envelope 5 along the length ofelectrodes 6 (although the material of lamp envelope and electrodes 6are not bound together). Sealing material 10 is kept at a distance fromthe interior of lamp envelope 5 so that sealing material 10 neverexperiences the high temperatures of the lamp. Solder glasses canachieve very high wetting but can flow at relatively low temperatures.Solder glasses typically are available in a finely ground powder orpaste, which can be placed at the desired location and then heated.Grading glasses, however, typically would have to be manipulated throughwell-known glass-blowing techniques.

To prevent chemical reaction between solder glass 10 and sulfur plasma,very long electrode feedthroughs may be used so that the maximumtemperature the solder glass is exposed to is kept to a safe lowerlimit.

FIGS. 3 and 4 show examples of such very long electrical leadfeedthroughs 12, bonded to a hermetic molybdenum-to-glass seal 14.Molybdenum-to-glass seal 14 is a very thin ribbon, typically a fewthousandths of an inch thick, embedded within the glass. Molybdenumribbon seals are a standard mechanism for achieving a seal, because whenthere is a mismatch of thermal expansion the molybdenum can yield to thestress and maintain a vacuum-tight seal. Molybdenum-to-glass seal 14does not just attach to feedthroughs 12 and the glass, but also createsa hermetic seal. Electrical lead feedthroughs 12 may pass into a blindhole bored into a silicon carbide electrode 7 to allow for themechanical insertion of electrical lead 12 as shown in FIG. 3, in whichcase electrical leads 12 may simply rest within silicon carbideelectrodes 7. Alternatively, electrical leads 12 may be soldered orbonded directly to silicon carbide electrodes 6 as shown in FIG. 4 usinga metallic solder, direct welding, or any electrically conductiveadhesive or solder, so long as there is enough contact for electricityto pass between electrical leads 12 and electrodes 6. Electrical leads12 may be made of tungsten, molybdenum, silver, nichrome, platinum, orgold.

With reference to FIG. 5, sealing of silicon carbide electrodes 6 tolamp envelope 5 may also be accomplished by using a tungsten sealingglass 11 (such as Corning 3320, which wets with silicon carbide) as anintermediate grading material between the silicon carbide and analuminosilicate glass or 33 expansion borosilicate glass such as Corning7740 or SCHOTT 8330. In this instance, envelope 5 is made of either thealuminosilicate glass or the 33 expansion borosilicate glass. However,the operating parameters of the electrode-driven sulfur plasma lamp arelimited in the instance where 33 expansion borosilicate glass is used.The temperature of the lamp when using a 33 expansion borosilicate glassenvelope is limited as operating temperature approaches the Tg of the 33expansion borosilicate glass, at which deformation or ballooning canoccur due to softening of the glass. Grading glass or intermediate glass11 is used to achieve a hermetic and mechanically robust seal betweenelectrodes 6 and lamp envelop 5. Ideally, intermediate glass 11 is aglass that wets silicon carbide electrode 6 easily and also wets thematerial of lamp envelope 5. In other words, intermediate glass 11achieves good interfacial adherence to silicon carbide electrodes 6 andto lamp envelope 5 at the hot temperatures at which sealing occurs dueto the ability of glass to flow at such temperatures. The technique ofFIG. 5 is advantageous if there is a substantial mismatch between thematerial of lamp envelope 5 and silicon carbide electrodes 6. Gradingglass 11 might be suitable for wetting and sealing the silicon carbide,but, for example, might not be transparent or might not have the rightmechanical strength to serve as lamp envelope 5. In certain embodiments,there might be more than one intermediate glass to progressively achievematching if there is a difference in thermal expansion characteristicsbetween the lamp envelope and the electrodes.

With reference to FIG. 6, a satisfactory seal between silicon carbideelectrodes 8 and glass envelope 5 is achievable by producing an undercutin the silicon carbide electrodes so that the electrodes have aninverted geometry formed in the body of the electrodes, and allowing theundercut to be filled with the glass in order to promote a moremechanically robust hermetic seal between silicon carbide electrode 8and glass envelope 5. Because of thermal expansion differences, thesilicon carbide will expand more upon heating and clamp down on orconstrict the glass, or the glass will expand more and clamp down on orconstrict the silicon carbide. Increased hermetic integrity results fromthe glass and silicon carbide pressing against each other upon heatingand cooling, ensuring that a strong bond is achieved. In order for thisseal to work properly, thermal expansion differences of the lampmaterial with respect to the silicon carbide electrodes must becarefully chosen. This mechanical hermetic seal may be the sole sealingmechanism, or hermetic sealing can also form due to bonding of the glassto silicon carbide at hot temperatures, with the mechanical hermeticsealing serving as an assist. FIG. 7 illustrates a similar embodiment inwhich silicon carbide electrodes 9 have an undercut forming an inwardprojecting feature that achieves the same effect as electrodes 8 in FIG.6.

Another last method used to seal the silicon carbide to the ceramicenvelope is by using a spark plasma sintering apparatus to force ahermetic seal between the silicon carbide and the aluminum oxide. Thistechnique is similar to the pinch seal technique described above, exceptthat it is performed under very high pressure and temperature. Aluminumoxide is a ceramic and may creep at high temperature, and so weldingoccurs under constant high pressure and high temperature with thealuminum oxide being under a static load. An aluminum oxide tube may beprovided, very near its final shape, and a rod may be inserted insidethe aluminum oxide tube, and then the pressure and temperature causesthe bonding.

Operation of the lamp is accomplished at an electrical potential of atleast 80 volts with a frequency of at least 50 hertz, although it ispossible a direct current could be used. Alternatively, higherfrequencies on the order of kilohertz, or even as much as a megahertz,offer greater efficiency in terms of light output per energy input tothe system due to the higher frequency excitation energy approaching thenatural rate the excited luminous states depopulate. e.g. less lightemitting transitions remain depopulated per unit time at higherexcitation frequencies. Voltage needed to drive the sulfur plasma lampwith the silicon carbide electrodes depends largely on fill pressurewithin the envelope and the distance between the electrodes.

The silicon carbide believed to exhibit the best performance as theelectrode material is an n-doped silicon carbide. N-doped siliconcarbide electrodes are able to drive the sulfur plasma directly andreliably. It is possible that p-doping will also be effective, or thatsilicon carbide can be used without doping. Because silicon carbide isvery chemically stable, exhibiting essentially no reaction with sulfurexcept at extremely high temperatures, any reaction with the sulfurplasma will be very slow.

There has been described a system and method for a plasma lamp and itsmanufacture and use. While several particular forms of the inventionhave been illustrated and described, it will be apparent that variousmodifications and combinations of the invention detailed in the text anddrawings can be made without departing from the spirit and scope of theinvention. Accordingly, it is not intended that the invention belimited, except as by the appended claims.

1. A plasma lamp, comprising: a lamp envelope of transparent ortranslucent glass or ceramic material; at least three electrodeshermetically sealed with the lamp envelope and in contact with aninterior of the lamp envelope, each of the at least three electrodesbeing substantially evenly spaced relative to a closest other one of theat least three electrodes; a quantity of material within the interior ofthe lamp envelope sufficient to create a plasma upon excitation; and abuffer gas within the interior of the lamp envelope for enabling initialdischarge and heating of the interior of the lamp envelope to excite thematerial into a plasma state; and a single-phase power supply connectedto each of the at least three electrodes.
 2. A method of operating aplasma lamp, comprising: providing a plasma lamp comprising: a lampenvelope of transparent or translucent glass or ceramic material; atleast three electrodes hermetically sealed with the lamp envelope and incontact with an interior of the lamp envelope, each of the at leastthree electrodes being substantially evenly spaced relative to a closestother one of the at least three electrodes; a quantity of materialwithin the interior of the lamp envelope sufficient to create a plasmaupon excitation; and a buffer gas within the interior of the lampenvelope for enabling initial discharge and heating of the interior ofthe lamp envelope to excite the material into a plasma state; providinga single-phase power supply connected to each of the at least threeelectrodes; and driving the plasma lamp by firing the power supplyacross different pairs of the at least three electrodes at differenttimes, repeatedly switching between the different pairs at a fixed rate.3. A method in accordance with claim 2 wherein the switching occurs at arate of at least a couple of dozen hertz.
 4. A method in accordance withclaim 3 wherein switching occurs in the kilohertz range.
 5. A method inaccordance with claim 3 wherein switching occurs in the megahertz range.6. A method in accordance with claim 2 wherein the repeated switchingbetween the different pairs of the at least three electrodes inducesstirring of the plasma.